Zoom lens system, imaging device and camera

ABSTRACT

A zoom lens system comprising a first lens unit of negative power, a second lens unit of positive power and a third lens unit of positive power, wherein in zooming, the lens units move respectively along an optical axis so that an interval of first lens unit and second lens unit decreases while an interval of second lens unit and third lens unit changes with variable magnification, the first lens unit comprises one object side negative lens element and one image side positive lens element, which have an aspheric surface, and the conditions: n12&gt;1. 88, ν12&lt;26 and 3.0&lt;f 3 /f W &lt;5.5 (ω W &gt;30, n 12  and ν 12  are refractive index and Abbe number, respectively, of the image side positive lens element of the first lens unit, f 3  is a composite focal length of the third lens unit, f W  is a focal length of the entire zoom lens system at a wide-angle limit) are satisfied.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/712,506,filed on Mar. 1, 2007 now U.S. Pat. No. 7,310,191, which claims priorityto application No. 2006-64710 filed in Japan on Mar. 9, 2006, thecontent of which is hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to a zoom lens system, an imaging deviceand a camera. In particular, the present invention relates to: a zoomlens system that has a high resolution, sufficient periphery illuminanceensured in the entire variable magnification range and high capabilityof compensating curvature of field, and that still has a short overalloptical length at the time of non-use; an imaging device employing thiszoom lens system; and a camera employing this imaging device.

2. Description of the Background Art

In the prior art, a large number of optical instruments have beendeveloped that form an image of a photographic object onto an imagesensor through a lens and then acquire the object image as an image.Recently, products such as digital still cameras and digital videocameras are spreading. Then, with the increase in the number of users,desire on these products is also growing. Among various types of theseproducts, optical instruments having a zoom ratio of approximately threeare comparatively small and still have an optical zoom function. Thus,these types are spreading remarkably widely as digital cameras ofcompact type or stylish type.

In the digital cameras of compact type, for the purpose of the propertyof easy carrying, further size reduction of the instruments is desired.In order to achieve the further size reduction of the digital cameras,the lens arrangement need be adopted such that the overall opticallength (the distance measured from the top of the most object side lenssurface of the entire lens system to the image surface) at the time ofnon-use should be reduced while lens elements that extend out relativeto the main body by means of a multi-stage lens barrel at the time ofuse could be accommodated into the main body.

Meanwhile, as zoom lens systems suitable for digital still cameras ofcompact type, a large number of zoom lens systems of three-unitconstruction have been proposed that, for example, in order from theobject side to the image side, comprise a first lens unit havingnegative optical power, a second lens unit having positive opticalpower, and a third lens unit having positive optical power.

In such a zoom lens system of three-unit construction, in zooming(magnification change) from a wide-angle limit to a telephoto limit, theair space between the first lens unit and the second lens unit decreasesmonotonically, while the air space between the second lens unit and thethird lens unit varies, and while the third lens unit is fixed or moved.

Focus adjustment in the zoom lens system of three-unit construction isperformed by moving the first-lens unit or the third lens unit in theoptical axis direction. In particular, from the perspective of sizereduction of the entire optical instrument, in many cases, the focusadjustment is performed using the third lens unit which is less heavy,so that focusing onto the photographic object is achieved ranging fromthe infinity to a short distance. In contrast, when the focus adjustmentis performed using the first lens unit, the first lens unit is largerthan the third lens unit and hence requires a large size motor. Thiscauses a tendency of size increase in the entire optical instrument.

The third lens unit having positive optical power has the effects ofcompensating curvature of field and bringing into a telecentric statethe incident light onto a light receiving surface of the image sensor.Further, in many cases, the third lens unit is constructed from one ortwo lens elements having a small outer diameter, and hence can be drivenat a high speed using a small size motor. Thus, when the third lens unitis adopted as a lens unit for focus adjustment, an optical instrument isrealized that has a reduced size and permits rapid focusing.

The first lens unit and the second lens unit move in parallel to theoptical axis along a cam groove formed in a cylindrical cam. In the camgroove, a groove for zooming and a groove for the time of non-use areconnected to each other. The groove for the time of non-use reduces theinterval between the lens units and moves all the three lens units tothe image sensor side. This configuration reduces the overall opticallength at the time of non-use. In this case, if the thickness of eachlens unit could be reduced, the overall optical length at the time ofnon-use would be reduced further.

As such, in the prior art, design has been performed such that the zoomlens system should have the above configuration where the size isreduced in the part relevant to focus adjustment and in the entire lenssystem at the time of non-use, so that the overall optical length of thedigital still camera has been reduced.

For example, Japanese Laid-Open Patent Publication No. 2005-134746discloses a three-unit zoom lens, in order from the object side to theimage side, comprising: a first lens unit having negative optical powerthat is composed of a negative-powered lens having an aspheric surfaceand a positive-powered lens; a second lens unit having positive opticalpower; and a third lens unit having positive optical power. In thisthree-unit zoom lens, the most object side negative-powered lens of thefirst lens unit is provided with a high refractive index, so that thelens thickness in the periphery part is reduced in a state thatcurvature of field at a wide-angle limit is compensated. This reducesthe thickness of the entire first lens unit and hence the size of theoptical system.

Further, for example, Japanese Laid-Open Patent Publication No.2005-084597 discloses a three-unit zoom lens that, in order from theobject side to the image side, comprises a first lens unit havingnegative optical power, a second lens unit having positive optical powerand provided with a diaphragm, and a third lens unit having positiveoptical power, wherein in magnification change, the first lens unitmoves relatively in a direction approaching to the second lens unit,while the second lens unit monotonically moves to the object side, andwhile the third lens unit moves to the object side and then movesreverse to the image side, and wherein when the object distance isinfinity, the position of the third lens unit at a wide-angle limit islocated on the object side relative to the position at a telephotolimit. In this three-unit zoom lens, a condition is set forth concerningthe focal length of the first lens unit in such a manner that thecompensation of curvature of field and the size reduction of the opticalsystem can be achieved simultaneously.

Further, new optical materials having refractive indices or Abbe numbersin the range unavailable in the past can recently be obtainedcomparatively easily. Thus, proposals have been made that such opticalmaterials are applied to the lenses included in a zoom lens so that sizereduction may be achieved in the optical system.

For example, Japanese Laid-Open Patent Publication No. 2006-011096discloses a zoom lens, in order from the object side to the image side,comprising: a first lens unit having negative optical power that iscomposed of a negative-powered lens having an aspheric surface and apositive-powered lens; a second lens unit having positive optical power;and a third lens unit having positive optical power, wherein the firstlens unit includes a lens element having a refractive index exceeding1.9.

Nevertheless, in the configuration of the three-unit zoom lens disclosedin Japanese Laid-Open Patent Publication No. 2005-134746, thepositive-powered lens on the image side of the first lens unit has a lowrefractive index and still is a spherical lens. This causes a problem ofinsufficiency in the compensation of curvature of field.

Further, in the configuration of the three-unit zoom lens disclosed inJapanese Laid-Open Patent Publication No. 2005-084597, for the purposeof size reduction, the focal length of the first lens unit is set uprather short. Nevertheless, in this case, although the diameter of thelens can be constructed comparatively small, when the first lens unit iscomposed of two lenses, the optical power becomes excessive in theobject side lens. Further, the thickness of the image side lens alsoincreases for the purpose of compensation of chromatic aberration. Thiscauses a problem of increase in the overall optical length at the timeof non-use.

Further, the configuration of the zoom lens disclosed in JapaneseLaid-Open Patent Publication No. 2006-011096 has a problem that theincident angle of light that is incident on the image sensor isexcessively large especially at a wide-angle limit. When the incidentangle becomes large as described here, a loss is caused in the amount oflight depending on shading characteristic of the image sensor, andthereby reduces the amount of periphery light in the shot image.Further, in the configuration of the zoom lens disclosed in JapaneseLaid-Open Patent Publication No. 2006-011096, fluctuation in theincident angle at the time of zooming is also large. This causes aproblem of large fluctuation in the seeing condition of the image in theperiphery part.

SUMMARY

An object of the present invention is to provide: a zoom lens systemthat has a high resolution, sufficient periphery illuminance ensured inthe entire variable magnification range and high capability ofcompensating curvature of field, and that still has a short overalloptical length at the time of non-use; an imaging device employing thiszoom lens system; and a camera employing this imaging device.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a zoom lens system for forming an optical image of an object with avariable magnification, in order from the object side to the image side,comprising: a first lens unit having negative optical power; a secondlens unit having positive optical power; and a third lens unit havingpositive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit decreaseswhile an interval between the second lens unit and the third lens unitchanges so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens elementwith the convex surface facing the object side; and one image sidepositive lens element with the convex surface facing the object sidethat is arranged with an air space on the image side of the object sidenegative lens element,

each of the two lens elements constituting the first lens unit has anaspheric surface, and

the following conditions (1), (2) and (3) are satisfied:n12>1.88  (1)ν12<26  (2)3.0<f ₃ /f _(W)<5.5  (3)(here, ω_(w)>30)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit,

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit,

f₃ is a composite focal length of the third lens unit,

f_(W) is a focal length of the entire zoom lens system at a wide-anglelimit, and

ω_(W) is a half view angle at the wide-angle limit.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an imaging device capable of converting an optical image of aphotographic object into an electric image signal and then outputtingthe signal, comprising:

a zoom lens system that forms the optical image of the photographicobject with a variable magnification; and

an image sensor that converts the optical image of the photographicobject formed by the zoom lens system into the electric image signal,wherein

the zoom lens system, in order from the object side serving as thephotographic object side to the image side, comprises: a first lens unithaving negative optical power; a second lens unit having positiveoptical power; and a third lens unit having positive optical power,

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit decreaseswhile an interval between the second lens unit and the third lens unitchanges so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens elementwith the convex surface facing the object side; and one image sidepositive lens element with the convex surface facing the object sidethat is arranged with an air space on the image side of the object sidenegative lens element,

each of the two lens elements constituting the first lens unit has anaspheric surface, and

the following conditions (1), (2) and (3) are satisfied:n12>1.88  (1)ν12<26  (2)3.0<f ₃ /f _(W)<5.5  (3)(here, ω_(W)>30)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit,

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit,

f₃ is a composite focal length of the third lens unit,

f_(W) is a focal length of the entire zoom lens system at a wide-anglelimit, and

ω_(W) is a half view angle at the wide-angle limit.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a camera capable of shooting a photographic object and then outputtingits image as an electric image signal, comprising:

an imaging device including a zoom lens system that forms the opticalimage of the photographic object with a variable magnification, and animage sensor that converts the optical image of the photographic objectformed by the zoom lens system into the electric image signal, wherein

the zoom lens system, in order from the object side serving as thephotographic object side to the image side, comprises: a first lens unithaving negative optical power; a second lens unit having positiveoptical power; and a third lens unit having positive optical power,

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit decreaseswhile an interval between the second lens unit and the third lens unitchanges so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens elementwith the convex surface facing the object side; and one image sidepositive lens element with the convex surface facing the object sidethat is arranged with an air space on the image side of the object sidenegative lens element,

each of the two lens elements constituting the first lens unit has anaspheric surface, and

the following conditions (1), (2) and (3) are satisfied:n12>1.88  (1)ν12<26  (2)3.0<f ₃ /f _(W)<5.5  (3)(here, ω_(W)>30)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit,

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit,

f₃ is a composite focal length of the third lens unit,

f_(W) is a focal length of the entire zoom lens system at a wide-anglelimit, and

ω_(W) is a half view angle at the wide-angle limit.

The present invention provides: a zoom lens system that has a highresolution, sufficient periphery illuminance ensured in the entirevariable magnification range and capability of satisfactory compensationof curvature of field, and that still has a reduced thickness of thefirst lens unit and a short overall optical length at the time ofnon-use; and an imaging device employing this zoom lens system. Thepresent invention further provides a small and high performance cameraemploying this imaging device.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of this invention will become clearfrom the following description, taken in conjunction with the preferredembodiments with reference to the accompanied drawings in which:

FIGS. 1A to 1C are configuration diagrams of a zoom lens systemaccording to Embodiment 1 (Example 1);

FIGS. 2A to 2I are longitudinal aberration diagrams of a zoom lenssystem according to Example 1;

FIGS. 3A to 3C are configuration diagrams of a zoom lens systemaccording to Embodiment 2 (Example 2);

FIGS. 4A to 4I are longitudinal aberration diagrams of a zoom lenssystem according to Example 2;

FIGS. 5A to 5C are configuration diagrams of a zoom lens systemaccording to Embodiment 3 (Example 3);

FIGS. 6A to 6I are longitudinal aberration diagrams of a zoom lenssystem according to Example 3;

FIGS. 7A to 7C are configuration diagrams of a zoom lens systemaccording to Embodiment 4 (Example 4);

FIGS. 8A to 8I are longitudinal aberration diagrams of a zoom lenssystem according to Example 4; and

FIG. 9 is a schematic construction diagram of a digital still cameraaccording to Embodiment 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 1 to 4

FIGS. 1A to 1C are configuration diagrams of a zoom lens systemaccording to Embodiment 1. FIGS. 3A to 3C are configuration diagrams ofa zoom lens system according to Embodiment 2. FIGS. 5A to 5C areconfiguration diagrams of a zoom lens system according to Embodiment 3.FIGS. 7A to 7C are configuration diagrams of a zoom lens systemaccording to Embodiment 4. Each of FIGS. 1A to 1C, 3A to 3C, 5A to 5C,and 7A to 7C shows a zoom lens system in an infinity in-focus condition.FIGS. 1A, 3A, 5A and 7A show the lens construction at a wide-angle limit(the shortest focal length condition: focal length f_(W)) FIGS. 1B, 3B,5B and 7B show the lens construction at a middle position (the middlefocal length condition: focal length f_(M)=√{square root over ()}(f_(W)*f_(T))). FIGS. 1C, 3C, 5C and 7C show the lens construction ata telephoto limit (the longest focal length condition: focal lengthf_(T)).

Each zoom lens system according to Embodiments 1 to 4, in order from theobject side to the image side, comprises: a first lens unit G1 havingnegative optical power; a diaphragm A; a second lens unit G2 havingpositive optical power; and a third lens unit G3 having positive opticalpower. In the zoom lens system according to Embodiments 1 to 4, inzooming from the wide-angle limit to the telephoto limit, the first lensunit G1 moves with locus of a convex to the image side, while the secondlens unit G2 and the diaphragm A monotonically move to the object side,and while the third lens unit G3 moves with changing the interval withthe second lens unit G2. That is, in the zoom lens system according toEmbodiments 1 to 4, in zooming from the wide-angle limit to thetelephoto limit, the lens units move respectively along the optical axisin such a manner that the interval between the first lens unit G1 andthe second lens unit G2 decreases while the interval between the secondlens unit G2 and the third lens unit G3 changes. Further, in each ofFIGS. 1A to 1C, 3A to 3C, 5A to 5C, and 7A to 7C, a straight line drawnon the rightmost side indicates the position of an image surface S. Onits object side, a plane parallel plate P such as an infrared cutfilter, a face plate of an image sensor or the like is provided.

As shown in FIGS. 1A to 1C, in the zoom lens system according toEmbodiment 1, the first lens unit G1, in order from the object side tothe image side, comprises two lens elements consisting of: a negativemeniscus object side negative lens element L1 with the convex surfacefacing the object side; and a positive meniscus image side positive lenselement L2 with the convex surface facing the object side that isarranged with an air space on the image side of the object side negativelens element L1. Each of the object side negative lens element L1 andthe image side positive lens element L2 has an aspheric image sidesurface.

Further, in the zoom lens system according to Embodiment 1, the secondlens unit G2, in order from the object side to the image side,comprises: a positive meniscus third lens element L3 with the convexsurface facing the object side; a negative meniscus fourth lens elementL4 with the convex surface facing the object side; a negative meniscusfifth lens element L5 with the convex surface facing the object side;and a bi-convex sixth lens element L6. Among these, the third lenselement L3 and the fourth lens element L4 are cemented with each otherand thereby constitute a positive cemented lens element, while the fifthlens element L5 and the sixth lens element L6 are cemented with eachother and thereby constitute a positive cemented lens element. Further,the third lens element L3 serving as the most object side lens elementof the second lens unit G2 has an aspheric object side surface.

Further, in the zoom lens system according to Embodiment 1, the thirdlens unit G3 comprises solely a positive meniscus seventh lens elementL7 with the convex surface facing the image side. The seventh lenselement L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 1, as shown in Table 13described later, the image side positive lens element L2 constitutingthe first lens unit G1 has a notably high refractive index. Thus, in theobject side negative lens element L1, the thickness at large light beamheight is easily ensured, so that the lens thickness can be reduced.Thus, in the zoom lens system according to Embodiment 1, the overalloptical length at the time of non-use is reduced.

As shown in FIGS. 3A to 3C, in the zoom lens system according toEmbodiment 2, the first lens unit G1, in order from the object side tothe image side, comprises two lens elements consisting of: a negativemeniscus object side negative lens element L1 with the convex surfacefacing the object side; and a positive meniscus image side positive lenselement L2 with the convex surface facing the object side that isarranged with an air space on the image side of the object side negativelens element L1. Each of the object side negative lens element L1 andthe image side positive lens element L2 has an aspheric image sidesurface.

In the zoom lens system according to Embodiment 2, the second lens unitG2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a positive meniscus fourth lens element L4 with theconvex surface facing the object side; a negative meniscus fifth lenselement L5 with the convex surface facing the object side; and abi-convex sixth lens element L6. Among these, the fourth lens element L4and the fifth lens element L5 are cemented with each other and therebyconstitute a cemented lens element. Further, the third lens element L3serving as the most object side lens element of the second lens unit G2has an aspheric object side surface.

In the zoom lens system according to Embodiment 2, the third lens unitG3 comprises solely a bi-convex seventh lens element L7. The seventhlens element L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 2, as shown in Table 13described later, the image side positive lens element L2 constitutingthe first lens unit G1 has a comparatively high refractive index. Thus,edge thickness is relatively easily ensured even if the lens centerthickness is reduced, so that the lens thickness can be reduced. Thus,in the zoom lens system according to Embodiment 2, the overall opticallength at the time of non-use is reduced.

As shown in FIGS. 5A to 5C, in the zoom lens system of Embodiment 3, thefirst lens unit G1, in order from the object side to the image side,comprises two lens elements consisting of: a bi-concave object sidenegative lens element L1; and a positive meniscus image side positivelens element L2 with the convex surface facing the object side that isarranged with an air space on the image side of the object side negativelens element L1. Each of the object side negative lens element L1 andthe image side positive lens element L2 has an aspheric image sidesurface.

Further, in the zoom lens system according to Embodiment 3, the secondlens unit G2, in order from the object side to the image side,comprises: a positive meniscus third lens element L3 with the convexsurface facing the object side; a negative meniscus fourth lens elementL4 with the convex surface facing the object side; a negative meniscusfifth lens element L5 with the convex surface facing the object side;and a bi-convex sixth lens element L6. Among these, the third lenselement L3 and the fourth lens element L4 are cemented with each otherand thereby constitute a positive cemented lens element, while the fifthlens element L5 and the sixth lens element L6 are cemented with eachother and thereby constitute a positive cemented lens element. Further,the third lens element L3 serving as the most object side lens elementof the second lens unit G2 has an aspheric object side surface.

Further, in the zoom lens system according to Embodiment 3, the thirdlens unit G3 comprises solely a positive meniscus seventh lens elementL7 with the convex surface facing the image side. The seventh lenselement L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 3, as shown in Table 13described later, the object side negative lens element L1 constitutingthe first lens unit G1 has a low refractive index, while the thicknessat large light beam height is small. However, the image side positivelens element L2 of the first lens unit G1 has a comparatively highrefractive index and an aspheric surface on the image side. Thus, evenwhen the thickness at large light beam height of the object sidenegative lens element L1 is small so that the compensation of distortionor curvature of field on the wide-angle limit side is insufficient, inthe entire zoom lens system according to Embodiment 3, the compensationeffect of the image side positive lens element L2 allows the image sidepositive lens element L2 to compensate sufficiently the distortion andthe curvature of field on the wide-angle limit side.

As shown in FIGS. 7A to 7C, in the zoom lens system according toEmbodiment 4, the first lens unit G1, in order from the object side to,the image side, comprises two lens elements consisting of: a negativemeniscus object side negative lens element L1 with the convex surfacefacing the object side; and a positive meniscus image side positive lenselement L2 with the convex surface facing the object side that isarranged with an air space on the image side of the object side negativelens element L1. Each of the object side negative lens element L1 andthe image side positive lens element L2 has an aspheric image sidesurface.

In the zoom lens system according to Embodiment 4, the second lens unitG2, in order from the object side to the image side, comprises: abi-convex third lens element L3; a bi-concave fourth lens element L4; anegative meniscus fifth lens element L5 with the convex surface facingthe object side; and a bi-convex sixth lens element L6. Among these, thethird lens element L3 and the fourth lens element L4 are cemented witheach other and thereby constitute a positive cemented lens element,while the fifth lens element L5 and the sixth lens element L6 arecemented with each other and thereby constitute a positive cemented lenselement. Further, the third lens element L3 serving as the most objectside lens element of the second lens unit G2 has an aspheric object sidesurface.

In the zoom lens system according to Embodiment 4, the third lens unitG3 comprises solely a bi-convex seventh lens element L7. The seventhlens element L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 4, the two lens elementsL1 and L2 constituting the first lens unit G1 contact with each other ina vicinity where the light beam from the object passes. Thus, thethickness of the entire first lens unit G1 can be reduced. Further, evenwhen the object side negative lens element L1 and the image sidepositive lens element L2 of the first lens unit G1 approach with eachother so that the capability of compensating distortion is degraded, thedistortion at the wide-angle limit is compensated satisfactorily in theentire zoom lens system according to Embodiment 4 since the image sidepositive lens element L2 is a lens element having a comparatively highrefractive index as shown in Table 13 described later, and an asphericsurface on the image side.

In the zoom lens system according to Embodiments 1 to 4, the lens unitsG1 to G3 are arranged in a desired optical power construction so thatsize reduction is achieved in the entire lens system in a state thatexcellent optical performance is satisfied.

In particular, in the zoom lens system according to Embodiments 1 to 4,the first lens unit G1 is constructed from: one object side negativelens element with the convex surface facing the object side; and oneimage side positive lens element with the convex surface facing theobject side. Further, the second lens unit G2 is constructed from twosets of positive cemented lens elements each fabricated by cementing twolens elements, or alternatively has such a construction that one set ofcemented lens element is placed between positive lens elements eacharranged on the object side or the image side. Furthermore, the thirdlens unit G3 is constructed from one lens element. As such, the zoomlens system according to Embodiments 1 to 4 realizes a lens system thathas a small number of lens elements constituting each lens unit and ashort overall optical length at the time of non-use.

As described above, in the zoom lens system according to Embodiments 1to 4, the second lens unit G2 is constructed from two sets of positivecemented lens elements or alternatively has such a construction that oneset of cemented lens element is placed between positive lens elementseach arranged on the object side or the image side. Instead, the secondlens unit G2 may, in order from the object side to the image side,comprise one set of positive cemented lens element and one positive lenselement, so that a lens system can be realized that has a short overalloptical length at the time of non-use.

In the zoom lens system according to Embodiments 1 to 4, each of theobject side negative lens element and the image side positive lenselement constituting the first lens unit G1 has an aspheric surface,while the image side positive lens element has a specific refractiveindex and a specific Abbe number. Thus, the zoom lens system accordingto Embodiments 1 to 4 has excellent optical performance, for example, incompensation of curvature of field.

Conditions are described below that are to be satisfied by a zoom lenssystem like the zoom lens system according to Embodiments 1 to 4, inorder from the object side to the image side, comprises a first lensunit having negative optical power, a second lens unit having positiveoptical power, and a third lens unit having positive optical power,wherein the first lens unit is constructed from: one object sidenegative lens element with the convex surface facing the object side;and one image side positive lens element with the convex surface facingthe object side, and wherein each of the two lens elements constitutingthe first lens unit has an aspheric surface. Here, a plurality ofconditions to be satisfied are set forth for the zoom lens systemaccording to each embodiment. The construction that satisfies all theconditions is most desirable for the zoom lens system. However, when anindividual condition is satisfied, a zoom lens system providing thecorresponding effect can be obtained.

For example, in a zoom lens system like the zoom lens system accordingto Embodiments 1 to 4, the following conditions (1), (2) and (3) aresatisfied;n12>1.88  (1)ν12<26  (2)3.0<f ₃ /f _(W)<5.5  (3)(here, ω_(W)>30)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit,

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit,

f₃ is a composite focal length of the third lens unit,

f_(W) is a focal length of the entire zoom lens system at a wide-anglelimit, and

ω_(W) is a half view angle at a wide-angle limit.

The conditions (1) and (2) set forth the refractive index and the Abbenumber of the image side positive lens element constituting the firstlens unit. When these conditions (1) and (2) are satisfied, the centerthickness of the image side positive lens element becomes small, whilecurvature of field on the wide-angle limit side is suppressed withoutthe necessity of a large curvature in the image side surface, so thatedge thickness is easily ensured. Thus, the thickness of the first lensunit can be reduced. This reduces the thickness of the entire zoom lenssystem and hence the overall optical length at the time of non-use.

The condition (3) sets forth an appropriate focal length of the thirdlens unit. When the value goes below the lower limit of the condition(3), the necessary optical power of the third lens unit becomes large.This causes a difficulty in compensation of spherical aberration or comaaberration in a variable magnification range where the third lens unitcomparatively approaches the object side. In contrast, when the valueexceeds the upper limit of the condition (3), the necessary opticalpower of the third lens unit becomes small. This causes an increase inthe amount of movement of the third lens unit, and hence causesdifficulty in size reduction of the optical system.

Here, when at least one of the following conditions (1)′ and (2)′ issatisfied, the above effect is achieved more successfully. When thefollowing condition (1)′ is satisfied, the image side positive lenselement of the first lens unit can have a large Z value (differencebetween curvature of the object side surface and curvature of the imageside surface), so that the centering of the lens becomes easier.Further, when the following condition (2)′ is satisfied, chromaticaberration generated in the first lens unit can be compensated moresatisfactorily.n12>1.95  (1)′ν12<24  (2)′

Here, when at least one of the following conditions (3)′ and (3)″ issatisfied, the above effect is achieved more successfully.3.8<f ₃ /f _(W)  (3)′f ₃ /f _(W)<4.4  (3)″

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingcondition (4) is satisfied;5.0<αi _(W)<20.0  (4)(here, ω_(W)>30)

where,

αi_(W) is an incident angle of the principal ray onto the image sensorat the maximum image height at a wide-angle limit (here, the incidentangle is defined as positive when the principal ray is incident on thelight receiving surface of the image sensor with departing from theoptical axis), and

ω_(W) is the half view angle at the wide-angle limit.

The condition (4) sets forth the incident angle of the principal rayonto the image sensor at the maximum image height at the wide-anglelimit. When the condition (4) is satisfied, the incident angle of themost off-axial principal ray that is incident onto the image sensorbecomes small. This reduces the influence of shading. When the valueexceeds the upper limit of the condition (4), a possibility arises thatthe influence of shading of the image sensor causes a decrease in theamount of periphery light. In contrast, when the value goes below thelower limit of the condition (4), the negative angle of the mostoff-axial principal ray at the telephoto limit becomes large at the timeof magnification change. This causes a possibility that the amount ofperiphery light is reduced especially at the telephoto limit.

Further, when the following condition (4)′ is satisfied, fluctuation inthe incident angle of the principal ray onto the image sensor at themaximum image height is suppressed in zooming and so is fluctuation inthe amount of periphery light. Thus, this configuration is remarkablyeffective.αi_(W)<15.0  (4)′

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingconditions (5), (6), (7) and (8) are satisfied;n11>1.50  (5)ν11>35  (6)n12−n11>0.10  (7)ν11−ν12>15.0  (8)

where,

n11 is a refractive index of the object side negative lens element ofthe first lens unit,

ν11 is an Abbe number of the object side negative lens element of thefirst lens unit,

n12 is the refractive index of the image side positive lens element ofthe first lens unit, and

ν12 is the Abbe number of the image side positive lens element of thefirst lens unit.

The conditions (5) and (6) set forth the refractive index and the Abbenumber of the object side negative lens element constituting the firstlens unit. The conditions (7) and (8) relate to conditions forperforming satisfactory compensation of chromatic aberration of a zoomlens system where the first lens unit is of negative-lead and hasnegative optical power while the first lens unit comprises an objectside negative lens element and an image side positive lens element. Whenthese conditions (5), (6), (7) and (8) are satisfied, a possibility isavoided that the optical axial thickness of the lens element increaseswith increasing light beam height and that when the center thickness isincreased for the purpose of improvement in manufacturability, thethickness of the entire first lens unit increases further. At the sametime, chromatic aberration can be compensated satisfactorily.

Further, when at least one of the following conditions (5)′, (6)′, (7)′and (8)′ is satisfied, the above effect is achieved more successfully.Furthermore, when at least one of the following conditions (6)″ and (8)″is satisfied, chromatic aberration generated in the first lens unit canbe compensated more satisfactorily.n11>1.75  (5)′ν11>38  (6)′65>ν11  (6)″n12−n11>0.12  (7)′ν11−ν12>17.5  (8)′45.0>ν11−ν12  (8)″

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingcondition (9) is satisfied;T1/Y<1.5  (9)

where,

T1 is a center thickness of the first lens unit, and

Y is the maximum image height.

The condition (9) sets forth the center thickness of the first lens unitin a zoom lens system where the first lens unit is of negative-lead andhas negative optical power, and hence easily becomes large. When thecondition (9) is satisfied, a possibility is avoided that the thicknessof the first lens unit increases excessively and so does the overalloptical length at the time of non-use.

Further, when the following condition (9)′ is satisfied, optical poweris imparted to the air lens in the first lens unit. Thus, thecompensation of curvature of field becomes easier on the wide-angleside.0.8<T1/Y  (9)′

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingcondition (10) is satisfied;(T1+T2+T3)/Y<3.5  (10)

where,

T1 is the center thickness of the first lens unit,

T2 is a center thickness of the second lens unit,

T3 is a center thickness of the third lens unit, and

Y is the maximum image height.

The condition (10) sets forth the total center thickness of the lensunits. When the condition (10) is satisfied, a possibility is avoidedthat the total thickness of the lens units increases excessively and sodoes the overall optical length at the time of non-use.

When the following condition (10)′ is satisfied, the above effect isachieved more successfully. Further, when the following condition (10)″is satisfied, the thickness of each lens unit, especially the thicknessof the first lens unit and the thickness of the second lens unit, can beensured. This permits more satisfactory compensation of curvature offield.(T1+T2+T3)/Y<3.2  (10)′2.5<(T1+T2+T3)/Y  (10)″

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingcondition (11) is satisfied;|αi _(W) −αi _(T)|<15.0  (11)(here, ω_(W)>30)

where,

αi_(W) is the incident angle of the principal ray onto the image sensorat the maximum image height at the wide-angle limit (here, the incidentangle is defined as positive when the principal ray is incident on thelight receiving surface of the image sensor with departing from theoptical axis),

αi_(T) is an incident angle of the principal ray onto the image sensorat the maximum image height at a telephoto limit (here, the incidentangle is defined as positive when the principal ray is incident on thelight receiving surface of the image sensor with departing from theoptical axis), and

ω_(W) is the half view angle at the wide-angle limit.

The condition (11) sets forth the incident angle of the principal rayonto the image sensor at the maximum image height at the time ofzooming. When the condition (11) is satisfied, fluctuation in theincident angle of the most off-axial principal ray that is incident ontothe image sensor becomes small, so that fluctuation in the seeingcondition of the image of the periphery part is maintained within anallowable range.

Here, when the conditions (4) and (11) are satisfied simultaneously, theilluminance of image periphery can be maintained appropriately. Thus,this configuration is remarkably preferable.

Here, the lens units constituting the zoom lens system of Embodiments 1to 4 are composed exclusively of refractive type lens elements thatdeflect the incident light by refraction (that is, lens elements of atype in which deflection is achieved at the interface between media eachhaving a distinct refractive index). However, the present invention isnot limited to the zoom lens system of this construction. For example,the lens units may employ diffractive type lens elements that deflectthe incident light by diffraction; refractive-diffractive hybrid typelens elements that deflect the incident light by a combination ofdiffraction and refraction; or gradient index type lens elements thatdeflect the incident light by distribution of refractive index in themedium.

Further, in the zoom lens system according to Embodiments 1 to 4, when areflecting surface may be arranged in the optical path so that theoptical path may be bent before or after the zoom lens system oralternatively in the middle. The bending position may be set uparbitrarily depending on the necessity. When the optical path is bentappropriately, thickness reduction in appearance can be achieved in acamera.

As described above, according to the present invention, a zoom lenssystem is obtained that compensates curvature of field satisfactorilyand that still has a reduced thickness of the first lens unit and ashort overall optical length at the time of non-use.

Embodiment 5

FIG. 9 is a schematic construction diagram of a digital still cameraaccording to Embodiment 5. In FIG. 9, the digital still cameracomprises: an imaging device including a zoom lens system 1 and an imagesensor 2 that is a CCD; a liquid crystal display monitor 3, and a body4. The employed zoom lens system 1 is the zoom lens system according toEmbodiment 1. In FIG. 9, the zoom lens system 1 comprises a first lensunit G1, a diaphragm A, a second lens unit G2, and a third lens unit G3.In the body 4, the zoom lens system 1 is arranged on the front side,while the image sensor 2 is arranged on the rear side of the zoom lenssystem 1. The liquid crystal display monitor 3 is arranged on the rearside of the body 4, while an optical image of a photographic objectacquired through the zoom lens system 1 is formed on the image surfaceS.

The lens barrel comprises a main barrel 5, a moving barrel 6, and acylindrical cam 7. When the cylindrical cam 7 is rotated, the first lensunit G1, the second lens unit G2, and the third lens unit G3 move topredetermined positions relative to the image sensor 2, so that variablemagnification can be achieved ranging from the wide-angle limit to thetelephoto limit. The third lens unit G3 is movable in the optical axisdirection by a motor for focus adjustment.

As such, when the zoom lens system according to Embodiment 1 is employedin a digital still camera, a small digital still camera is obtained thathas a high resolution and high capability of compensating the curvatureof field and that has a short overall optical length at the time ofnon-use. Here, in the digital still camera shown in FIG. 9, any one ofthe zoom lens systems according to Embodiments 2 to 4 may be employed inplace of the zoom lens system according to Embodiment 1. Further, theoptical system of the digital still camera shown in FIG. 9 may beapplied to a digital video camera for moving images. In this case,moving images with high resolution can be acquired in addition to stillimages.

An imaging device comprising a zoom lens system according to Embodiments1 to 4 described above and an image sensor such as a CCD or a CMOS maybe applied to a mobile telephone, a PDA (Personal Digital Assistance), asurveillance camera in a surveillance system, a Web camera, avehicle-mounted camera or the like.

Hereinafter, numerical examples which are actual implementations of thezoom lens systems according to Embodiments 1 to 4 will be described. Inthe numerical examples, the units of the length in the tables are all“mm”. The units of the angle in the tables are all “degree”. Moreover, ris the radius of curvature, d is the axial distance, nd is therefractive index to the d-line, and νd is the Abbe number to the d-line.In the numerical examples, the surfaces marked with * are asphericalsurfaces, and the sag z of the aspherical surface is defined by thefollowing expression:

$z = {\frac{{ch}^{2}}{1 + \sqrt{\left\{ {1 - {\left( {1 + k} \right)c^{2}h^{2}}} \right\}}} + {A\; h^{4}} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10} + {Eh}^{12}}$Here, h is the height from the optical axis, c is the curvature, k isthe conic constant, and A, B, C, D and E are the fourth-order,sixth-order, eighth-order, tenth-order and twelfth-order asphericalcoefficients, respectively.

EXAMPLE 1

A zoom lens system of Example 1 corresponds to Embodiment 1 shown inFIGS. 1A to 1C. Table 1 shows the lens data of the zoom lens system ofExample 1. Table 2 shows the aspherical data. Table 3 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d11 and d13, when the shootingdistance is infinity.

TABLE 1 Lens Lens unit element Surface r d nd νd G1 L1  1 51.824 1.1001.805 41.0 *2 6.287 2.433 L2  3 12.887 1.400 2.400 17.0 *4 17.651Variable Diaphragm  5 ∞ 0.300 G2 L3 *6 4.617 1.900 1.805 41.0 L4  730.191 0.500 1.717 29.5  8 4.069 0.600 L5  9 19.289 0.600 1.620 36.3 L610 4.413 1.200 1.589 61.3 11 −12.764 Variable G3 L7 12 −237.873 1.1001.665 55.2 *13  −15.297 Variable P 14 ∞ 0.900 1.517 64.2 15 ∞ 0.870

TABLE 2 Surface k A B C D E 2 −3.612E−01   7.170E−06 −3.185E−06−1.903E−09  −1.340E−09 0.000E+00 4 0.000E+00 −9.258E−05  1.154E−060.000E+00  0.000E+00 0.000E+00 6 0.000E+00 −5.397E−04 −1.839E−05 1.169E−07 −3.148E−08 0.000E+00 13 0.000E+00  4.422E−04 −5.274E−056.216E−06 −3.426E−07 7.090E−09

TABLE 3 Axial Wide-angle Middle Telephoto distance limit position limitd4 14.79 8.07 2.37 d11 2.70 9.09 19.06 d13 6.10 4.47 2.64 f 5.36 8.8116.71 F-number 2.98 3.86 5.66 2ω 70.01 44.60 23.97 L 36.50 34.53 36.96

EXAMPLE 2

A zoom lens system of Example 2 corresponds to Embodiment 2 shown inFIGS. 3A to 3C. Table 4 shows the lens data of the zoom lens system ofExample 2. Table 5 shows the aspherical data. Table 6 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d12 and d14, when the shootingdistance is infinity.

TABLE 4 Lens Lens unit element Surface r d nd νd G1 L1  1 97.238 1.3001.878 38.2 *2 6.116 2.181 L2  3 15.696 1.778 1.996 20.5 *4 62.842Variable Diaphragm  5 ∞ 0.300 G2 L3 *6 4.711 1.500 1.804 40.8  7 20.9310.300 L4  8 8.092 0.800 1.697 55.5 L5  9 52.433 0.400 1.805 25.5 103.521 0.419 L6 11 24.775 0.993 1.697 55.5 12 −24.775 Variable G3 L7 1333.551 1.438 1.518 70.3 *14  −15.270 Variable P 15 ∞ 0.900 1.517 64.2 16∞ 0.870

TABLE 5 Surface k A B C D E 2 −7.285E−01  1.632E−04 −1.177E−05 3.548E−07 −1.538E−09  0.000E+00 4  0.000E+00 −1.619E−04  7.984E−06−2.917E−07 2.109E−09 0.000E+00 6 −1.425E−01 −4.078E−04  1.138E−05−7.290E−06 7.546E−07 0.000E+00 14  0.000E+00  2.119E−04 −1.102E−05 1.904E−07 1.254E−08 −4.126E−10 

TABLE 6 Axial Wide-angle Middle Telephoto distance limit position limitd4 16.24 6.86 1.68 d12 2.70 7.90 17.30 d14 4.39 3.93 2.60 f 4.83 8.7916.50 F-number 2.99 3.96 5.91 2ω 75.75 45.08 24.67 L 36.58 31.92 34.84

EXAMPLE 3

A zoom lens system of Example 3 corresponds to Embodiment 3 shown inFIGS. 5A to 5C. Table 7 shows the lens data of the zoom lens system ofExample 3. Table 8 shows the aspherical data. Table 9 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d11 and d13, when the shootingdistance is infinity.

TABLE 7 Lens Lens unit element Surface r d nd νd G1 L1  1 −84.423 1.1001.514 63.3 *2 5.404 2.309 L2  3 14.387 1.600 1.900 24.0 *4 26.719Variable Diaphragm  5 ∞ 0.300 G2 L3 *6 4.964 1.900 1.805 41.0 L4  7144.593 0.500 1.717 29.5  8 4.496 0.600 L5  9 43.557 0.600 1.620 36.3 L610 6.034 1.200 1.589 61.3 11 −10.835 Variable G3 L7 12 −237.873 1.1001.665 55.2 *13  −14.202 Variable P 14 ∞ 0.900 1.517 64.2 15 ∞ 0.870

TABLE 8 Surface k A B C D E 2 −6.222E−01   2.697E−04 −9.608E−06−1.996E−07  4.805E−09 0.000E+00 4 0.000E+00 −2.732E−04  9.958E−06−1.370E−07  0.000E+00 0.000E+00 6 0.000E+00 −5.198E−04 −1.810E−05 1.721E−06 −1.330E−07 0.000E+00 13 0.000E+00  4.072E−04 −5.130E−05 6.943E−06 −4.141E−07 9.128E−09

TABLE 9 Axial Wide-angle Middle Telephoto distance limit position limitd4 14.38 8.04 2.03 d11 2.70 9.14 19.40 d13 6.45 4.83 2.60 f 5.57 8.8216.70 F-number 2.94 3.77 5.56 2ω 67.94 43.94 23.71 L 35.61 34.10 36.11

EXAMPLE 4

A zoom lens system of Example 4 corresponds to Embodiment 4 shown inFIGS. 7A to 7C. Table 10 shows the lens data of the zoom lens system ofExample 4. Table 11 shows the aspherical data. Table 12 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d11 and d13, when the shootingdistance is infinity.

TABLE 10 Lens Lens unit element Surface r d nd νd G1 L1  1 96.707 1.1001.878 38.2 *2 5.757 1.219 L2  3 9.382 1.778 1.996 20.5 *4 22.204Variable Diaphragm  5 ∞ 0.300 G2 L3 *6 4.225 1.500 1.805 41.0 L4  7−50.000 0.600 1.717 29.5  8 3.642 0.600 L5  9 15.017 0.600 1.620 36.3 L610 5.586 1.500 1.589 61.3 11 −16.364 Variable G3 L7 12 100.000 1.1001.665 55.2 *13  −15.520 Variable P 14 ∞ 0.900 1.517 64.2 15 ∞ 0.870

TABLE 11 Surface k A B C D E 2 −7.802E−01  5.551E−05  1.727E−07−1.998E−07  6.955E−09 0.000E+00 4  0.000E+00 −4.846E−05  3.420E−06 4.006E−08 −4.178E−09 0.000E+00 6  0.000E+00 −6.412E−04 −4.796E−05 6.036E−06 −6.470E−07 0.000E+00 13  0.000E+00  7.442E−04 −9.347E−05 9.894E−06 −5.255E−07 1.088E−08

TABLE 12 Axial Wide-angle Middle Telephoto distance limit position limitd4 16.77 7.10 2.17 d11 2.70 8.24 17.94 d13 4.97 4.43 2.64 f 4.75 8.8116.43 F-number 2.78 3.66 5.40 2ω 76.50 44.06 24.09 L 36.51 31.84 34.82

Table 13 shows values corresponding to the conditions in Examples 1 to4.

TABLE 13 Example Condition 1 2 3 4 (1) n12 2.40 2.00 1.90 2.00 (2) ν1217.0 20.5 24.0 20.5 (3) f₃/f_(w) 4.6 4.2 4.1 4.3 (4) αi_(w) 12.0 12.510.3 8.8 (5) n11 1.81 1.88 1.51 1.88 (6) ν11 41.0 38.2 63.3 38.2 (7) n12− n11 0.60 0.12 0.39 0.12 (8) ν11 − ν12 24.0 17.7 39.3 17.7 (9) T1/Y1.37 1.46 1.39 1.14 (10)  (T1 + T2 + T3)/Y 3.01 3.09 3.03 2.78 (11)  |αi_(w) − αi_(T) | 10.2 12.3 9.2 7.7 T1 4.93 5.26 5.01 4.10 T2 4.80 4.414.80 4.80 T3 1.10 1.44 1.10 1.10 T1 + T2 + T3 10.83 11.11 10.91 10.00 Y3.60 3.60 3.60 3.60

FIGS. 2A to 2I are longitudinal aberration diagrams of a zoom lenssystem according to Example 1. FIGS. 4A to 4I are longitudinalaberration diagrams of a zoom lens system according to Example 2. FIGS.6A to 6I are longitudinal aberration diagrams of a zoom lens systemaccording to Example 3. FIGS. 8A to 8I are longitudinal aberrationdiagrams of a zoom lens system according to Example 4.

FIGS. 2A to 2C, 4A to 4C, 6A to 6C, and 8A to 8C show the longitudinalaberration at the wide-angle limit. FIGS. 2D to 2F, 4D to 4F, 6D to 6F,and 8D to 8F show the longitudinal aberration at an approximate middleposition. FIGS. 2G to 2I, 4G to 4I, 6G to 6I, and 8G to 8I show thelongitudinal aberration at the telephoto limit. FIGS. 2A, 2D, 2G, 4A,4D, 4G, 6A, 6D, 6G, 8A, 8D and 8G are spherical aberration diagrams.FIGS. 2B, 2E, 2H, 4B, 4E, 4H, 6B, 6E, 6H, 8B, 8E and 8H are astigmatismdiagrams. FIGS. 2C, 2F, 2I, 4C, 4F, 4I, 6C, 6F, 6I, 8C, 8F and 8I aredistortion diagrams. In each spherical aberration diagram, the verticalaxis indicates the F-number, and the solid line, the short dash line andthe long dash line indicate the characteristics to the d-line, theF-line and the C-line, respectively. In each astigmatism diagram, thevertical axis indicates the half view angle ω, and the solid line andthe dash line indicate the characteristics to the sagittal image plane(in each Fig., indicated as “s”) and the meridional image plane (in eachFig., indicated as “m”), respectively. In each distortion diagram, thevertical axis indicates the half view angle ω.

The zoom lens system according to the present invention is applicable toa camera such as a digital still camera, a digital video camera, amobile telephone, a PDA (Personal Digital Assistance), a surveillancecamera in a surveillance system, a Web camera or a vehicle-mountedcamera. In particular, the present zoom lens system is suitable for acamera such as a digital still camera or a digital video camerarequiring high image quality.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention, theyshould be construed as being included therein.

1. A zoom lens system, in order from an object side to an image side,comprising: a first lens unit having negative optical power, movingalong an optical axis during zooming, the first lens unit, in order fromthe object side to the image side, having a negative lens element with aconvex surface facing the object side and a positive lens element with aconvex surface facing the object side; a second lens unit havingpositive optical power, moving along an optical axis during zooming, thesecond lens unit having two sets of positive cemented lens elements; anda third lens unit having positive optical power, moving along an opticalaxis during zooming, wherein the following conditions (1), (2) and (3)are satisfied:n12>1.88  (1)ν12<26  (2)3.0<f₃/f_(W)<5.5  (3)(where, ω_(w)>30) in which, n12 is a refractive index of the positivelens element of the first lens unit, ν12 is an Abbe number of thepositive lens element of the first lens unit, f₃ is a composite focallength of the third lens unit, f_(W) is a focal length of the entirezoom lens system at a wide-angle limit, and ω_(W) is a half view angleat a wide-angle limit.
 2. The zoom lens system as claimed in claim 1,satisfying the following condition (4):5.0<αi_(W)21 20.0  (4)(where, ω_(W)>30) wherein, α i_(W) is an incident angle of the principalray onto the image sensor at the maximum image height at a wide-anglelimit (here, the incident angle is defined as positive when theprincipal ray is incident on the light receiving surface of the imagesensor with departing from the optical axis), and ω_(W) is a half viewangle at a wide-angle limit.
 3. The zoom lens system as claimed in claim1, satisfying the following conditions (5), (6), (7) and (8):n11>1.50  (5)ν11>35  (6)n12−n11>0.10  (7)ν11−ν12>15  (8) wherein, n11 is a refractive index of the negative lenselement of the first lens unit, ν11 is an Abbe number of the negativelens element of the first lens unit, n12 is a refractive index of thepositive lens element of the first lens unit, and ν12 is an Abbe numberof the positive lens element of the first lens unit.
 4. The zoom lenssystem as claimed in claim 1, satisfying the following condition (9):T1/Y<1.5  (9) wherein, T1 is a center thickness of the first lens unit,and Y is the maximum image height.
 5. The zoom lens system as claimed inclaim 1, satisfying the following condition (10):(T1+T2T3)/Y<3.5  (10) wherein, T1 is a center thickness of the firstlens unit, T2 is a center thickness of the second lens unit, T3 is acenter thickness of the third lens unit, and Y is the maximum imageheight.
 6. The zoom lens system as claimed in claim 1, wherein the thirdlens unit is composed of one lens element.
 7. An imaging device capableof converting an optical image of a photographic object into an electricimage signal and then outputting the signal, comprising: a zoom lenssystem that forms the optical image of the photographic object with avariable magnification; and an image sensor that converts the opticalimage of the photographic object formed by the zoom lens system into theelectric image signal, wherein the zoom lens system, in order from theobject side serving as the photographic object side to the image side,comprises: a first lens unit having negative optical power, moving alongan optical axis during zooming, the first lens unit, in order from theobject side to the image side, having a negative lens element with aconvex surface facing the object side and a positive lens element with aconvex surface facing the object side; a second lens unit havingpositive optical power, moving along an optical axis during zooming, thesecond lens unit having two sets of positive cemented lens elements; anda third lens unit having positive optical power, moving along an opticalaxis during zooming, and the zoom lens system satisfies the followingconditions (1), (2) and (3:n12>1.88  (1)ν12<26  (2)3.0<f₃/f_(W)<5.5  (3)(where, ω_(W)>30) in which, n12 is a refractive index of the positivelens element of the first lens unit, ν12 is an Abbe number of thepositive lens element of the first lens unit, f₃ is a composite focallength of the third lens unit, f_(W) is a focal length of the entirezoom lens system at a wide-angle limit, and ω_(W) is a half view angleat a wide-angle limit.
 8. The imaging device as claimed in claim7,wherein the zoom lens system satisfies the following condition (4):5.0<αi_(W)<20.0  (4)(where, ω_(W)>30) wherein, α i_(W) is an incident angle of the principalray onto the image sensor at the maximum image height at a wide-anglelimit (here, the incident angle is defined as positive when theprincipal ray is incident on the light receiving surface of the imagesensor with departing from the optical axis), and ω_(W) is a half viewangle at a wide-angle limit.
 9. The imaging device as claimed in claim7, wherein the zoom lens system satisfies the following conditions (5),(6), (7) and (8):n11>1.50  (5)ν11>35  (6)n12−n11>0.10  (7)ν11−ν12>15.0  (8) wherein, n11 is a refractive index of the negativelens element of the first lens unit, ν11 is an Abbe number of thenegative lens element of the first lens unit, n12 is a refractive indexof the positive lens element of the first lens unit, and ν12 is an Abbenumber of the positive lens element of the first lens unit.
 10. Theimaging device as claimed in claim 7, wherein the zoom lens systemsatisfies the following condition (9):T1/Y<1.5  (9) wherein, T1 is a center thickness of the first lens unit,and Y is the maximum image height.
 11. The imaging device as claimed inclaim 7, wherein the zoom lens system satisfies the following condition(10):(T1+T2+T3)/Y<3.5  (10) wherein, T1 is a center thickness of the firstlens unit, T2 is a center thickness of the second lens unit, T3 is acenter thickness of the third lens unit, and Y is the maximum imageheight.
 12. The imaging device as claimed in claim 7, wherein the thirdlens unit of the zoom lens system is composed of one lens element.